We consider a 3-dimensional slab, with width \(L\)  in comoving coordinates
along the \(x\) -direction and width \({\tilde{L}}\rightarrow \infty \)  along the \(y\) - and \(z\) -directions.
The physical width of the slab grows proportionally to \(a(\tau )\) ,
following the general expansion.
The space inside the slab is entangled with the exterior. The corresponding
entanglement entropy at a given time \(T\)  can be computed through the
extension of the Ryu-Takayanagi proposal [1]}, [2]}
given in ref. [3]}.
The entropy is proportional to the area
\({\rm Area}(\gamma _A)\equiv I(\epsilon )={\tilde{L}}^2\int _\epsilon dx\frac{A^2(\tau (x),z(x))}{z^3(x)}\sqrt{A^2(\tau (x),z(x))-N^2(\tau (x),z(x)){\left( {\frac{d\tau (x)}{dx}} \right)}^2+{\left( {\frac{dz(x)}{dx}} \right)}^2},\) 
